I’d be interested to see a comparison between a long and short mast head crane, with regards to forestay tension.
Basic upwind performance lies in good headstay tension regardless of sail profile…
I use a long crane, coupled with a deep and stiff mast profile, and a 4:1 purchase on the back stay to achieve what I consider good tension… I think, from memory the rig has either 9 or 7 mm deflection with 1000g weight in the centre with the supports 1700 mm apart, I will check on Monday when I return to work as my ‘black book’ with the numbers is there.
A bend test is useful in checking the basic stiffness of the rig.
A big head main might not be what you want upwind, but it is quite the reverse downwind… So there a two sides to the coin. The key is managing and utilising twist to control the high C of E in gusts upwind to unleash the power downhill!
It was necessary to review the Servo-Arm setting because I decided to use the full swing of the servo of 120° obtained with Joystick+trim. Is not a usual method, but this is the limitation imposed by the Savöx 1256 combined with the transmitter Hitec Laser 4.
here the updating drawing :
This modification imply to use a shorter Arm from 87mm to 77mm and therefore getting more power available.
As a second talk, I’m thinking about the future 123 development consisting in the use of the drum servo RMG 280. Is expensive but offer the correct power and sheeting length. Battery power may be reconsidered too.
ClaudioD
PS :
Because of the “V” Spreaders, the Main Boom cannot open at 85°/90°, but only at 77°. This condition force to run off wind at an angle above 13°/14°. No problems since generally the course should be at least 20° off wind to get more speed !
This made me think: why the V spreader in the first place? To position the ancher point at a larger beam? To relief some tension on the back-stay? To favour/help/create the bending of the top third of the mast?
this system is rather recent on real boats, the main advantage is that the runners are eliminated and changing direction is much more simplified when maneuvering the Genoa.
The mast need to be of stronger construction and pre-bent. This is not my case.
On real boats carbon mast are a must, while heavier then classic system.
Technically, the high shrouds push the mast toward the bow while the lower shrouds are pulling the mast toward the back. When finally well “trimmed”, (not easy), the results are producing a very stable mast without deformations under external forces.
So at the end, on real boats, the runners are not needed anymore and the mast is more stable.
I’m not sure will be worth in my case, but I like to try.
Unfortunately, there is a limitation on the main boom opening when running. Personally I will not suffer of that since when running I sail off the wind to get more speed !
Finally I got in troubles when I started the final sheeting assembly, before the final Deck/Hull bonding.
First, with the shorter arm I made, the measured sheet length came out to be 255mm instead of 265mm. See drawing at post 301.
Two reasons: in the sheet length calculations it is necessary to add the bloc diameter. In my case is 10mm, thus 255 +10 = 265mm. Probably, I was wrong, when measuring only the distance from the bloc centers at close hauled and running positions.
There is another imprecision source is that the servo rotation is not 120° (joystick+trim) but only 118°.
A second problem was due to the friction coming from the sheet interference with the bloc mounting axis.
Because of that, I was obliged to orientate the bloc as such to avoid these interferences and designing a New Arm.
Anchor point will close to the Main fairlead to reduce the enter/exit sheeting angles to the blocs
It is not excluded to substitute the blocs with eye bolts in spite of increased friction ! I’m in conflict with the two methods !!!
Hi John
probably I shall go back to the arm equipped with floating or pivoting bloc as I did in the past with an RG65.
HS-815 is much too heavy !!! and often burn out if contrasted too long as on my AC33 !!
Thank for the infos.
ClaudioD
looks like your new arm should do the trick with the poulie - I would always prefer a poulie to an eyebolt for less friction - having said that I don’t have a poulie on the top of the mainsheet post the main sheet wear a bit by going through off course but I find that I don’t have to replace before the next season. So I get a year out of it and even then I’m sure I can keep it longer but I don’t take the chance to have it break on my in the middle of a race. Your eyebolt looks quite smooth so the wearing should be minimum.
With my last tests sequence probably I found the solution with the pivoting dual pulleys, as I did in the past with the RG65 with one single pulley.
Still I have to fix definitively the anchoring vertical position since the two pulley stay at different level (both sides of the Arm) and avoid any vertical friction with the pulley shoulders.
A 4th arm need to be done tomorrow and pass a new order for pulleys to Cap Maquette !
Suitable pulleys in the picture .
Why all that troubles ?
The Savox 1256 TG servo power of 20kgcm at 6V. may be still not sufficient to control easily the 78dm² of sail with winds over 12kt, this is the reason to reduce all possible frictions !!
What you smoking there Claudio ! (joke) Never heard of any AC 120’s running this high servo power with same sail area ? but thinking drums here not arms (no experince) does the arm reduce power with length ?
Hi Alan,
Yes of course the arm length reduce dramatically the power available at booms level
With 78dm² sail area the servo arm of 85mm long need to be more then 25kgcm to cope with 12/15kt. My servo offer only 20kgcm.
With troubles experienced with sheeting length and various frictions, I spent my night thinking to a better solution.
My conclusion is : or I use a powerful servo or for the immediate conclusion of this prototype, to recover from one of Class M the old HS725BB servo winch that is not very powerful, only 13.8kgcm and it is slow, but better of the actual servo arm Savox 1256, fast but weak and noisy.
I spent this morning reconsidering the full design and I introduced the RMG 280. Surprisingly is offering a very good option at the buoy !!! pity because rather expensive for my pocket !
Some pics of the modifications !
See attached revision with servo winch Hitec and future outcome budget with RMG 280.
ClaudioD
Hull interior cleaned and ready to start again. Only one mast step/tube provision, if something wrong during tuning, I will modify the sail plan as it is done with real boats. Some additional picture of the dismount exercise. To note that the hull for 431g was before painting, while the pictures of today include the paint estimated to 15g.
CD
Absent for some times while busy to find the Servos Winch Set Up solution after abandonment of the Servo Arm and with particular attention for the Jib sheeting circuit.
RMG servo solution is seriously considered for the next model.
It is not excluded also to modify the actual Savox 1256 for a multi turns Servo Drum using an external potentiometer.
Finally got the solution with the help of Alan with various pm exchanges on the subject.
A new Dual Fair-lead is under finalization , see pictures.
No post for 3 days. You had us all worried for you :).
Please take into account that I have never seen a servo winch and I have no experience in advanced digital RC.
According to the HS-725BB datasheet, the rotational range is 1260°, three turn and a half. Since I don’t see any limit switchs on boats using a servo winch, I always assumed that you need to carefully calculate the diameter of the drum in order to get the proper linear movement for sheeting.
But then, I see in the datasheet that it takes 1,5s for a 60° turn of the winch, with no load(!) at 4,8V and 1,3s at 6V. That means half a minute for a complete gybe!!! What am I missing? I remember that you said once that the drawback of a “cheap” winch was its lack of speed. But that much?
[IMPORTANT EDITING] OK, the datasheet I had found had a typo. Sorry for the commotion. I see now that it is 1,5s/360°. Much better, but still slow at more than 5 secondes for a gybe. Well, thinking about it, unless I want to see the boom swinging full speed over my head, it takes me at least that much on my catamaran. I more real lif experience then, which is kind of nice. But what about racing against swing arm boat? Or swing armed boat, I should say!
Bonjour Sylvain,
the situation is not so dramatic as you described in spite of the fact that this old servo is very slow and I explain why :
1- the Main sheet length is only 265mm since I cannot open the Main boom over 73° due to the shrouds fixed behind the mast and because of the ‘V’ shaped spreaders.
2- the Drum internal diameter is 38mm, therefore the circumference is 119.3mm
3 - the number of revolutions required are 265 / 119.3 = 2.22 rev.
4 - due to the speed of 1.3"(no load)/360°, the time required to develop 2.22 revolutions and 265mm sheet is : 1.3 x 2.22 = 2.88 seconds. Practically it may take some 3.5 seconds under load each time you want to go from running to close hauled.
Mentally count 3.5second and imagine turning around the buoy, you will realize that is not very much ! The maneuver need to be anticipated before reaching the buoy !
5 - The number of revolutions are adjusted on the transmitter acting on the ATV potentiometer adjustment trim, but you can also put a mechanical ‘stop’ on the joystick.
About the use of a swing rig is of course possible, but in this case the Mast step shall be advanced toward the bow , I guess, for about 4 to 5cm. I cannot be more precise since it shall be tested with the sail plan you want to use.
I don’t know if I’m missing something and correct me if I’m wrong, but looking at photo 467 as the main is sheeted in its attachment point to the drum circuit moves aft. Bearing this in mind, I wondered if it was really necessary for the jib sheet to run back to the fairlead on the main sheeting post & then run forward again with the additional associated friction. Would it not be better (ie more efficient) if the jib sheet attached to the drum circuit at the same point as the main and was then routed forward and then out through a fairlead and onto the jib boom?
As I say, I may have missed something and therefore welcome your thoughts…
Hi Row,
how is it now ? hope you fill better .
You do not miss anything, just I modified the Jib sheeting method other then the servo type.
The original design was with two fairleads one for the Main and one for the Jib in front of the Mast steps.
Looking closely to one of the previous drawings you may see how closed to the deck was the Jib exit almost flush. Under these conditions the risk of water leak was very high as was also mentioned by Alan and agreed upon.
The low fairlead exit is justified by the fact that the Jib boom shall stay as close as possible to the deck in order to increase the Jib efficiency avoiding thus the exchanges of air flush from one side to the other of the jib surface.
In view of the peculiarity of the situation, I decided, after some talks with Alan, to introduce a sheeting route with an additional “C” shaped tube and a “Z” tube on the roof, as can be seen in the pictures above.
The bent tubes, if well aligned with sheet entrance/exit, offer very little friction/drag.
This is the explanation for the changes.
Keep well
The actual measured weight with all the elements is :
1055g completed hull with mast, jib boom, 2 servos , battery 6V., receiver, spreaders, rudder links, jib struts.
Missing parts are :
Fin 133g
Rudder 35g
Sails 55g
S/S rigging harness 25g
Bulb 2920g
Total expected weight at the buoy is : 4223g .
Some saving may be reached substituting the HS725BB (110g) servo with Eurgle (65) and the battery of 6V with 4.8v, the weight will drop to 4158g against the design weight of 4129g.
The good new is that finally the weight is close to the design in spite of all changes, the bad new is that I’m still far from the ‘ideal’ weight of 4000g/4050g.
120g too much
ClaudioD
PS:
BTW, due to the measured hull volume of 3853g against the design of 3673g there is still a positive margin.
Hi Row, I think your question was misunderstood so I will try to explain, please correct me if I’m wrong Claudio !
All sheeting is preferred below deck & out of sight by Claudio, when you study the revised plan above post # 313 (update 12 July2012) with the return pulley mounted on the rudder servo bridge you can see ciruit/sheeting length is set at 265 mm.
At full running, the sheeting position point stops beside the winch drum “forward of the main sheeting post” hence a dual pulley has been introduced (mounted on keel box bridge) that guides sheeting to the sheeting post from the “front” hence the U shape sheet guide on sheeting post.
The only way to feed the sheeting post from the "rear with dual “F” guide on sheeting post (eliminating the front feed & U shaped sheeting guide) is to move the return pulley on the rudder servo bridge back to rudder horn bridge, but here is no space being so close to the transom.
(joke) Never heard of any AC 120’s running this high servo power with same sail area ? but thinking drums here not arms 